Ceramic metal halide light sources are comprised of a ceramic discharge vessel (commonly referred to as an arc tube) that is generally made of polycrystalline-alumina. Typical metal halide fills may include mercury, alkali- and alkaline-earth iodides, in particular NaI and CaI2, and rare-earth iodides such as DyI3, TmI3 and HoI3. Xenon or argon are typical gas fills. Tungsten electrodes are used to generate an arc within the discharge vessel. Because electrical power must be supplied to the electrodes, the electrode assemblies must extend through the arc tube wall. In a conventional construction, capillary tubes hold the electrode assemblies and a frit material is used to form a hermetic seal between the electrode assembly and its respective capillary. The ceramic arc tube is often jacketed in another envelope, called an outer jacket, to protect the metal parts from oxidation. These outer jackets are usually thermally isolated from the arc tube and contain a vacuum or are filled with a partial pressure of an inert gas and a getter material, e.g., an aluminum or zirconium compound, to getter hydrogen and oxygen.
In recent years, ceramic metal halide light sources have become increasingly favored because of their efficiency and color rendering properties. As a result, the applications for ceramic metal halide light sources have expanded into traditional incandescent lighting applications, such as parabolic reflector (PAR) lamps, which must be adapted to accommodate these high-intensity, high-temperature light sources. For example, a typical failure mode for ceramic metal halide sources occurs as a result of chemical attack by the metal halide fills on the frit materials used to make the electrode seals. In a conventional reflector lamp structure, this problem is exacerbated because some of the emitted visible radiation is reflected back onto the ceramic metal halide source, and in particular, the electrode seal located in the neck portion of the reflector. The construction of the electrode assembly and the seal make it a particularly good absorber of visible light. The absorbed energy causes the electrode seal to overheat which in turn increases the rate of chemical attack by the fill on the seal leading to premature lamp failure. Therefore, it would be advantageous to keep the electrode seal from overheating in order to extend the operating life of the lamp. It would be a further advantage to accomplish this without significantly affecting the performance or cosmetic appearance of the reflector lamp.